Hi Jules,
I've been making the migration to mac recently myself. I have OS 10.5.2 running on Intel.
If the source of your distribution is not an issue, I would recommend using
matplotlib-0.91.2-py2.5-macosx-10.3-fat.egg
from the matplotlib site, along with the easy_installer script.
That went fine for me.
Thanks
-Kaushik